One or more embodiments relate to a nonvolatile memory device and a method of operating the same.
In recent years, there has been an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and which do not require rewriting data at specific refresh intervals.
A typical nonvolatile memory cell of the nonvolatile memory device is configured to enable electrical program/erase operations and to perform the program and erase operations by varying a threshold voltage when electrons are moved by a strong electric field applied to a thin oxide layer.
A typical nonvolatile memory device includes a memory cell array in which cells for storing data are arranged in a matrix form and a page buffer for writing data into specific cells of the memory cell array or reading data stored in a specific cell. The page buffer may include a bit line pair coupled to a specific memory cell, a register configured to temporarily store data to be written into the memory cell array or to read data stored in a specific memory cell of the memory cell array and temporarily store the read data, a sense node configured to sense the voltage level of a specific bit line or a specific register, and a bit line selection unit configured to control whether or not to couple the specific bit line with the sensing node.
The threshold voltages of programmed cells can be changed due to the retention characteristic, disturbance, etc. of a nonvolatile memory device. The nonvolatile memory cell may have a low threshold voltage because electrons stored in the floating gate of the cell are discharged unintentionally due to the leakage current, etc. with a time lapse. A characteristic as to retention of programmed data during a time lapse is referred to as “the retention characteristic.” If the retention characteristic is poor, there is a concern in that programmed data may be read differently from the actual programmed data. In particular, when cells have several distributions of the threshold voltages as in a multi-level cell (MLC) program method, such a concern regarding the retention characteristic may become more noticeable because the read margin between the cells is relatively small. Furthermore, threshold voltage distributions of the cells can be changed by disturbance in the threshold voltage according to the program, erase, and read operations of neighboring cells.
Accordingly, when a read operation is performed on memory cells, it is desirable to change read voltages in response to a determination of how much threshold voltage distributions have been changed.